1. Field of the Invention
The present invention relates to a liquid crystal electro-optical device and a method for forming the same. Particularly, the present invention relates to a liquid crystal electrooptical device, and particularly, to a structure which is capable of maintaining a constant distance between the substrates (inter-electrode spacing) of a large area liquid crystal electrooptical device.
2. Prior Art
As a conventional liquid crystal electro optic device, TN type or STN type one utilizing nematic liquid crystal is widely known and is put to practical use. Recently, one utilizing ferroelectric liquid crystal is also known. Such liquid crystal electro optic device comprises: a pair of substrates faced with each other, with one substrate having an electrode on its surface and the second substrate having an electrode on its surface; a liquid crystal layer holding a liquid crystal material between these substrates; and utilizes electro optic effect caused by change of condition of liquid crystal molecules, said change caused by voltage applied to said liquid crystal layer, by anisotropy of dielectric constant of the liquid crystal material itself, or by intrinsic polarization in the case of utilizing antiferroelectric liquid crystal.
In the liquid crystal electro optic device of TN or STN type, liquid crystal molecules are arranged in the direction of rubbing on the interface between the substrates and the liquid crystal layer, caused by the force of rubbing performed for orientation treatment. In the substrates upside and downside, the directions of rubbing are crossed by 90.degree., or 200.degree. to 290.degree.. As a result, liquid crystal molecules around the middle of the liquid crystal layer are arranged spirally, making energy the smallest between molecules upside and downside placed by 90.degree. to 290.degree.. In this case, a kiral material is included in a liquid crystal material of STN liquid crystal if necessary.
In the liquid crystal electro optic device of TN or STN type above mentioned, the liquid crystal molecules arranged spirally unfasten this spiral structure by being arranged in parallel or vertically to the direction of field, by dielectric anisotropy of the liquid crystal molecules caused by voltage applied to both substrates. The device shows bright state when the liquid crystal molecules are arranged vertical to the substrate, and dark state when the liquid crystal molecules are arranged in parallel. The conditions of the liquid crystal molecules change continuously by changing voltage applied between the substrates. Therefore gradation display can be made by appropriately controlling voltage applied.
In the liquid crystal electro optic device utilizing ferroelectric liquid crystal or antiferroelectric liquid crystal, liquid crystal molecules are arranged in accordance with the force controlled by rubbing treatment, at least on one side of the substrate. These liquid crystal molecules have a layer structure in which layers are regularly deposited one after another, from one side of the substrate to the other side of the substrate.
In above mentioned liquid crystal electro optic device utilizing ferroelectric liquid crystal or antiferroelectric liquid crystal, the liquid crystal molecules which are arranged in parallel to the substrate in a layer structure change the direction of intrinsic polarization by 180.degree., which the liquid crystal molecules themselves have (hereinafter called as inversion). This inversion is caused by applying voltage between both substrates. The device changes the direction of the liquid crystal molecules which are arranged along the rubbing direction to some degree, by the inversion of the liquid crystal molecules. Thus switching is performed from bright state to dark state, or from dark state to bright state.
In what is called active matrix type liquid crystal electro optic device, further improved display with high speed, high brightness, and many gradations is performed. An active matrix electro optic liquid crystal device comprises a plurality of pixels on one side of the substrates holding a liquid crystal material and switching devices connected to each pixel, such as diode and thin film transistor (TFT).
A general structure of cell of above mentioned liquid crystal electro optic device is explained by FIG. 1. What is described is a device of simple matrix type. Electrode patterns 1102 and 1103 are formed on translucent substrates 1100 and 1101 such as glass or resin. A means of orientation 1104 and 1105 are formed on the electrodes to arrange the liquid crystal in one direction. The spacer 1107 distributed on the substrate keeps the minimum distance between the substrates. A seal material fixes the two substrates. A liquid crystal material 1106 is implanted between the substrates, thus forming a liquid crystal layer.
When a polyimide film subjected to rubbing treatment is used as a means to establish the molecular orientation, horizontally oriented liquid crystal molecules arranged along one direction, either in parallel with or making a predetermined angle with respect to the surface of the substrate, can be obtained. When a silane coupling agent is used, on the other hand, perpendicularly oriented liquid crystal molecules can be found to be arranged along a direction vertical to the surface of the substrate. The molecular orientation of the liquid crystal, which is either perpendicular or horizontal, depends on the desired mode of operation.
A voltage is applied to the liquid crystals by the electrodes 1102 and 1103. The liquid crystal molecules arrange themselves in correspondence with the electric field which is generated as a consequence of the applied voltage to provide a light state and a dark state. A uniform display can be obtained by maintaining a constant distance between the electrodes, and moreover, the two substrates must be placed at a constant distance. Though the distance between the electrodes differs depending on the operation mode, however, it is generally in the range of from 1.2 to 20 .mu.m with a precision of 0.05 .mu.m or less.
Recently, a large-area display is getting demanded for a liquid crystal electro optic device. Efforts have been made to embody a huge liquid crystal display with diagonal distance as long as 40 inches.
Though the size of the display is enlarged in this case, a distance demanded between substrates is as very short as 1.2 to 20 .mu.m as above mentioned. Accuracy of the display is also demanded as 0.05 .mu.m or less.
The weight of above mentioned display with 40-inch diagonal distance will be over 8 kg, just by facing glass substrates of 120 cm and 1.1 mm thickness. If the distance between the substrates is 6 .mu.m, the weight of liquid crystal put between the substrates amounts to 60 g. With such a large size, even though strong glass substrates are used, the substrates are wound like paper by the cause of their own weight, when the substrates are just held vertically.
Thus, it follows that a liquid crystal cell obtained by allowing the facing two substrates to stand straight upward undergoes deformation to yield a pear-like shape. Then, the distance between the electrodes at the lower portion of the cells increases to about several times to several tens of times, and, in extreme cases, several hundred times as large as that of the upper portion, thereby making it impossible to realize a uniform display due to the failure of generating a constant applied electric field.
In the case of a ferroelectric liquid crystal, moreover, the liquid crystal molecules are oriented as such to form a layered structure. Accordingly, the deformation of a substrate destroys the layered structure to impart a fatal damage to the liquid crystal, thereby making it unfeasible to make a display. This phenomenon is readily encountered in small displays having a display area as small as about 5 cm.sup.2, and it has been known as a serious problem in ferroelectric liquid crystal displays.
As described referring to FIG. 1 above, a conventional cell comprises a sealing material to fix the two substrates with a constant spacing. Since the inner portions are supported only by the spacers, it is quite natural that a severe deformation form more frequently with increasing distance from the sealed portions.
As a means to overcome the aforementioned problems of cell deformation, it has been proposed to adhere the two substrates by not only placing an adhesive at the peripheral sealing portions, but also scattering adhesive grains inside the cell. However, on the other hand, the adhesive grains caused disordering in the molecular orientation.
What decides the condition of liquid crystal molecules between substrates is mainly the strength of voltage applied to electrodes of both substrates. Impurities with charge from the liquid crystal or orientation films may exist, and unneeded charge producing voltage in opposite direction to the voltage applied may be produced in the device of the prior art, which problem is widely known.
This charge moves in the liquid crystal layer held between both substrates freely, by the cause of voltage applied. Most of this charge moves and reaches the surface of the orientation film, but charge will not move further because the orientation film is essentially insulative. Thus charge is accumulated between the orientation film and the liquid crystal layer (at the interface between the orientation film and liquid crystal).
This charge produces undesirable problem as a liquid crystal electro optic device. For example, effect to eliminate voltage applied between both substrate is made. In turning the liquid crystal molecules in an opposite direction enough, it is necessary to make the applied voltage bigger than the voltage needed in reversing intrinsic polarization. In applying voltage between the electrodes, the state of liquid crystal molecules is not stabilized because the amount of charge in the liquid crystal layer changes with passage of time. Moreover, the liquid crystal molecules electrically absorbed by the cause of charge accumulated at the interface between the orientation film and the liquid crystal in changing its state needs a voltage bigger than that of liquid crystal molecules which are not absorbed and in the liquid crystal layer. Therefore the liquid crystal molecules in the liquid crystal layer will not cause change of state at a time. This causes a problem that character of translucency of light is not stabilized, said character being the most important in the character of liquid crystal electro optic device.
To solve this problem, there is a method to select material for an orientation film relaxing accumulation of charge or a method to orient liquid crystal molecules by depositing SiO.sub.2 and the like on the electrode diagonally, instead of selecting an orientation film being an insulating film. However, these are not a common method because they are time-consuming for many preparative experiments, which results in high cost, and because effect may change by combination of materials. There is also a method of removing impurities by purifying the liquid crystal. However, this method is far from desirable when considering mass productivity because liquid crystal that can be used by purification is very little.
There is also a method to make charge of + or - as .+-.0 by absorbing or combining charge existing in the liquid crystal layer by the use of charge transfer complex. (Hereinafter called as cancel or neutralize) However, it is difficult to measure and take in charge transfer complex of an appropriate amount in canceling charge perfectly. If charge transfer complex in the liquid crystal layer is not enough, undesirable charge cannot be canceled. Excess charge transfer complex will move in the liquid crystal layer like above mentioned charge does, and causes defects.
As above mentioned, there are various kinds of methods suggested to cancel charge existing in a liquid crystal layer, said charge causing change of voltage applied to the liquid crystal layer, that is, causing change of condition of liquid crystal molecules with passage of time and making optical characteristic of the device unstable. However, a method which can cancel charge easily and perfectly has not existed yet.
The present invention is aimed at solving various kinds of problems as above mentioned. That is, the present invention provides a liquid crystal electro optic device which can make substrates adhered to each other without disordering orientation condition of the liquid crystal material, and manufacturing method thereof. The present invention is aimed at removing effects caused by undesirable charge in the liquid crystal layer to stabilize optic characteristic of the device, and is aimed at providing a high quality liquid crystal electro optic device without flicker or change in tone.